Add feature flag and boilerplate for commit list refactor

What does this MR do and why?

• Adds a new feature flag project_commits_refactor for commit list refactor work
• Adds a new entry point from app/views/projects/commits/show.html.haml to initiate a new Vue app
• Set up a Vue root component (commit_list_app.vue) with basic structure
• Create placeholder components for:
  • commit_list_header.vue
  • commit_list_item.vue

References

• proposed FE architecture - &17482

Screenshots or screen recordings

How to set up and validate locally

• Enable project_commits_refactor feature flag
• Go to a project
• Select Code > Commits
• Should see a basically empty app

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #536165 (closed)